Nice question, i asked it as well before :), so i first wanted to shout someting like "use the super search", or somthing like that...
Anyway, how can you search for this, if you don't know what to search for, so i'm sorry for that thought :)...
Anyway, as CountZero explained, Catalyst uses it like following:

<shameless quoting>
Catalyst needs those attributes to decide which sub to call when it parses the URL. You can see it working very nicely if you run the included your_app_server.pl script. In its debug-output you see it building your packages and subs linked to the URLs it understands.
</shameless quoting>

"We all agree on the necessity of compromise. We just can't agree on when it's necessary to compromise." - Larry Wall.